Chiral bound states in the continuum (BICs) play a crucial role in manipulating circularly polarized waves with ultrahigh quality (Q) factors. However, intrinsic chirality has predominantly been achieved through symmetry-protected BICs, which require structural symmetry breaking. Here, we propose a universal yet simple design strategy to achieve intrinsic chirality using accidental BICs. By tuning the air hole radius of the photonic crystal or introducing an external magnetic field to break time-reversal $\left(\mathcal{T}\right)$ symmetry, the doubly degenerate accidental BICs with topological charge −1 can transition into quasi-BICs. Consequently, the high-Q quasi-BICs can be realized without breaking structural symmetry. Under an external magnetic field, the nonradiative vortex singularities (V points) split into a pair of circularly polarized states (C points) at the Γ point, enabling the realization of intrinsic chirality that preserves structural symmetry. In addition, these chiral quasi-BICs can be further tuned into magneto-optical (MO) BICs by tailoring the air hole radius of the photonic crystal. More interestingly, the location of the MO BICs can be accurately determined by theoretically solving the characteristic equation of the coupling Hamiltonian. As a result, we further demonstrate switchable nonreciprocal transmission for circularly polarized waves at a given magnetic field strength. This work enriches the current research on BICs, significantly advancing the design of chiral photonic devices that are magnetically perturbed but still structurally symmetric.

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• Bound states in the continuum
• Chirality
• Magneto-optical effect
• Non-reciprocal transmission